1. Field of the Invention
The present invention relates to a gas injection system of a nuclear power plant for injecting hydrogen and oxygen in a nuclear power plant and also relates to a gas injection method therefor.
2. Description of the Related Prior Art
Generally, there has been known a technique of reducing an oxygen concentration of water to be brought into contact with a stainless steel as a technique for controlling a stress corrosion cracking of the stainless steel. In a nuclear power plant, fixed quantities of hydrogen and oxygen are generated in a reactor by radiolysis of water. By the generation of hydrogen and oxygen in the reactor, a dissolved oxygen concentration of the reactor water is determined based on the temperature-pressure conditions, and the dissolved oxygen concentration is usually about 100-200 ppb. The radiolysis of water is controlled and the oxygen concentration in the reactor is lowered by the injection of the hydrogen into the reactor water.
In the reactor, before the injection of the hydrogen, the hydrogen and oxygen in a main steam are sent in a volume ratio of 2:1, and after passing through a main condenser, they are recombined by a recombiner of a gaseous water disposal system and then become water to be recovered.
However, in case of injecting the hydrogen into the reactor, the oxygen is reduced due to the reduction of the quantity of the radiolysis of water in the reactor, and the hydrogen is injected from the outside, so that the volume ratios of the hydrogen and the oxygen in the main steam is not 2:1 and the quantity of the hydrogen extremely exceeds. Because of this reason, since an excess of the hydrogen is flowed into the gaseous waste disposal system, it is necessary to inject the oxygen so as to be recombined with the hydrogen.
However, in such a case as described above, a problem of quantity of gases to be injected is caused. That is, the quantity of the hydrogen to be injected varies in accordance with an output power of the nuclear power plant and is usually in the quantity of about 30-120 Nm.sup.3 /Hr. Almost all quantity of the injected hydrogen is flowed into the gaseous waste disposal system.
A flow rate of the exhaust gas which can be treated by the gaseous waste disposal system is about 20-40 Nm.sup.3 /Hr, which corresponds to the quantity for treating an air leaked from the main condenser. When an excess of hydrogen is flowed into the air, some portion thereof is recombined with the oxygen in the air, but the most portion thereof is flowed into the gaseous waste disposal system. The flow rate of the exhaust gas of the gaseous waste disposal system becomes 40-130Nm.sup.3 /Hr, exceeding the allowable flow rate.
Therefore, when the oxygen for the recombination with the hydrogen is injected into the gaseous waste disposal system, the hydrogen is recombined with the oxygen, and only the air leaked from the main condenser is treated in the gaseous waste disposal system.
Since the quantities of the hydrogen and the oxygen to be injected are larger than the allowable flow rate in the gaseous waste disposal system, it is an important matter to maintain a balance of the quantities of the hydrogen and the oxygen at an inlet of the gaseous waste disposal system.
Furthermore, since the hydrogen injected to a water system reaches the gaseous waste disposal system by way of the reactor, the main steam pipe and the main condenser of the nuclear power plant, there causes a time lag that elapses between the injection of the hydrogen and the reaching thereof to the gaseous waste disposal system, and it is difficult to define an injection timing of the oxygen to be injected into the gaseous waste disposal system, thus providing a problem.
Still furthermore, the hydrogen becomes an explosion area when it exceeds 4% in a volume ratio in a mixing condition with another gases. Accordingly, it is also important to avoid the condition where the hydrogen exceeds in its quantity.